A major challenge for mammalian hosts is to maintain symbiotic relationships with the vast bacterial communities that colonize the intestines. The intestinal epithelium is the primary barrier between the intestinal microbiota and internal host tissues, yet little is known about how epithelial cells control bacterial interactions with the mucosal surface and limit microbial penetration of the intestinal barrier. We have discovered a novel antibacterial factor, RegIIIg, which is produced by small intestinal epithelial cells. RegIIIg is a member of the C-type lectin family of carbohydrate-binding proteins that is discharged into the gut lumen. RegIIIg has direct bactericidal activity against Gram-positive bacteria and binds its bacterial targets via interactions with peptidoglycan carbohydrate. RegIIIg thus represents a previously unappreciated mechanism of mucosal defense and a new biological function for C-type lectins. Intestinal bacteria regulate RegIIIg transcription through direct activation of epithelial cell Toll-like receptors, revealing a direct dialog between enteric bacteria and the intestinal epithelium that regulates antimicrobial activity. As several other Reg family C-type lectins are produced by gut epithelia, directly bactericidal C-type lectins likely represent an important general mechanism of gut epithelial defense. The overall goal of this proposal is to extend our understanding of lectin- mediated antimicrobial defense of the intestinal mucosal surface. This work will involve three Specific Aims. First, we will use in vitro biochemical approaches to determine the molecular mechanism of lectin-mediated bactericidal activity. Second, we will use RegIIIg knockout mice to delineate how RegIIIg contributes to intestinal host-microbial homeostasis in vivo. Third, we will analyze other Reg proteins in vitro and in vivo in order to explore whether directly bactericidal lectins constitute a general mechanism of mucosal defense. Together, these studies should lead to fundamental insights into immune control of bacterial-mucosal associations, and will provide new perspectives on symbiotic host-microbial associations.